The present invention relates to the field of telecommunications systems in general and synchronous digital hierarchy (SDH) networks in particular.
Telecommunications operators commonly arrange network elements (NE) or nodes such as telephone exchanges into ring topology networks in which each node is connected to its immediate neighbours by either two or four fibres. An example of such a ring network is a ring where a multiplex section shared protection is deployed (MS-SPRING). Such networks need to allow for the possibility of equipment failure: either node failure or failure of a span between two adjacent nodes. In order to cater for such failures, the spans commonly comprise working channels for carrying normal traffic plus additional, so-called protection channels which allow for the re-routing of traffic in the event of a failure. These protection channels may be unused normally or carry extra traffic which can be discarded in the event of a failure. Routing of traffic via a protection channel in order to avoid a failure point in a ring is known as protection switching.
One problem with protection switching is what to do with traffic which can no longer be delivered to the correct destination due to such a failure. This can occur if the destination node has failed or if a number of spans have failed rendering the destination node out of reach of the rest of the network. A destination node which has failed or which is isolated from the rest of the network due to failure in one or more spans is said to be invalid. Traffic that is destined for an invalid node is said to be undeliverable. There is a danger that such undeliverable traffic may be misdirected and end up at the wrong destination. This could have serious consequences, particularly if confidential information were to fall into the wrong hands as a result. There is therefore a need to delete undeliverable traffic. The deletion of undeliverable traffic is called squelching.
In order to provide squelching a method is needed to enable undeliverable traffic to be identified. A proposed method for dealing with undeliverable traffic in MS-SPRING is described in ITU-T Standard G.841. MS-SPRING provides for the identification of undeliverable traffic by having a so-called node map at each node of the ring. The node map would contain information on the traffic passing through the corresponding node: its origin, destination and whether it is to be dropped in that node or to be passed through.
A problem with the use of node maps is that they place a considerable burden on the telecommunications operator who has to generate and maintain complex tables of traffic information. A second problem is that MS-SPRING as currently defined does not support low order virtual container (LOVC) access. A further and important disadvantage of MS-SPRING is that it does not support timeslot interchange (TSI). This places a severe limitation on operators and raises a question as to the practicality of using MS-SPRING in a commercial network.
The use of the present invention allows the above problems to be addressed and in particular allows the above problem of undeliverable traffic to be addressed without the disadvantages of node maps.
The present invention provides a synchronous, digital hierarchy (SDH) telecommunications system for the transport of traffic, said traffic comprising control bytes for indicating the sources of the traffic, said system comprising a plurality of interconnected nodes, the nodes arranged for the identification of undeliverable traffic by checking the indications of the sources.
Preferably the SDH telecommunications system also comprises a network management system (NMS) in which the NMS is arranged to provide each node with information concerning the expected sources of the traffic, in which the system is arranged to identify undeliverable traffic by means of a comparison of the sources indicated by the control bytes with the expected sources,
In a preferred embodiment the plurality of interconnected nodes comprise a multiplex section shared protection ring (MS-SPRING).
Each node may comprise a plurality of pieces of transmission equipment, each piece equipment may comprise a plurality of ports.
An example of a control byte is an overhead byte such as SDH path overhead (POH).